Path control method for multihop wireless network

Incorporation by reference

The present application claims priority from Japanese Patent Application No. 2011-202237, filed on Sep. 15, 2011 in Japan, the disclosure of which is incorporated herein by reference in its entirety.

Technical field

The present invention relates to a network system composed of a plurality of wireless nodes for carrying out multihop wireless communications with specific wireless nodes, and to a path control method therefor.

Background art

There are wireless sensor network systems as the above type of network systems. In a wireless sensor network system, on generating a path with a sink node as the destination and a sensor node as the source by applying a routing protocol suitable for wireless multihop networks such as OLSR (Optimized Link State Routing), AODV (Ad hoc On-Demand Distance Vector), and the like, as a result, a network topology of tree structure (to be referred to as tree topology hereinbelow) is constructed with a sink node as the apex and sensor nodes positioned at the portions of knots and leaves. In this case, however, bias may occur in the number of child nodes among a plurality of nodes of the same hierarchy. Since such kind of bias causes an increase of load on specific nodes, it is necessary to remove it as far as possible.

The following Patent Document 1 discloses an example of the path control method for removing such bias occurring in the tree topology as described above. In this technique, if a specific device (the network construction device 30 in FIG. 1 of the Patent Document 1) detects that a bias is occurring in the generated tree topology, in order to remove the bias, the system analyzes between which nodes connection should be changed, and carries out connection change after sending a message to each of the nodes as the objects of connection change.

[Patent Document 1]

Jp 2010-166150 a

However, in the method for carrying out connection change after sending a message to each node of a connection change object, it is necessary to interchange messages of transmission and response in proportion to the node number of the connection change objects.

Summary

An exemplary object of the present invention is to provide a path control method for multihop wireless networks, a multihop wireless network, and wireless nodes to solve the above problem, that is, a considerable communication cost is needed for improvement in removing the bias of tree topology in multihop wireless network systems.

In order to solve the above problem, an aspect in accordance with the present invention provides a path control method for multihop wireless network composed of a plurality of wireless nodes, the method including: calculating a metric value indicating a quantity of providing a guideline for selecting a superordinate destination node with each candidate for the superordinate destination node when carrying out a multihop wireless communication with a specific wireless node among the plurality of wireless nodes based on peripheral node information about the node information received from peripheral wireless nodes, by each of the wireless nodes; determining the superordinate destination node based on the calculated metric value, by each of the wireless nodes; selecting, as a target wireless node, the wireless node expected to change the number of the wireless nodes existing in a subordinate hierarchy lower than its own node from a network topology of tree structure with the specific wireless node as the root and other wireless nodes than the specific wireless node as the knots and leaves in the multihop wireless network; and changing, intentionally, a value of the node information sent from the target wireless node to the peripheral wireless nodes.

Further, another aspect in accordance with the present invention provides a multihop wireless network including: a plurality of wireless nodes; and a control node, wherein each of the wireless nodes includes: a peripheral node information acquisition unit for sending node information about its own wireless node to the peripheral wireless nodes while acquiring peripheral node information about the node information received from the peripheral wireless nodes, a metric calculation unit for calculating a metric value indicating a quantity of providing a guideline for selecting a superordinate destination node with each candidate for the superordinate destination node when carrying out a multihop wireless communication with a specific wireless node among the plurality of wireless nodes based on the acquired peripheral node information, and a superordinate destination node determination unit for determining the superordinate destination node based on the calculated metric value; and wherein the control node includes; a network management unit for generating a network topology of tree structure with the specific wireless node as the root and other wireless nodes than the specific wireless node as the knots and leaves in the multihop wireless network, selecting, as a target wireless node, the wireless node expected to change the number of the wireless nodes existing in a subordinate hierarchy lower than its own node from the generated network topology of tree structure, and changing, intentionally, a value of the node information sent from the target wireless node to the peripheral wireless nodes.

Further, still another aspect in accordance with the present invention provides a wireless node connected to a multihop wireless network, the wireless node including: a peripheral node information acquisition unit for sending node information about its own wireless node to peripheral wireless nodes while acquiring peripheral node information about the node information received from the peripheral wireless nodes; a metric calculation unit for calculating a metric value indicating a quantity of providing a guideline for selecting a superordinate destination node with each candidate for the superordinate destination node when carrying out a multihop wireless communication with a specific wireless node among the plurality of wireless nodes based on the acquired peripheral node information; a superordinate destination node determination unit for determining the superordinate destination node based on the calculated metric value; and a network management unit for generating a network topology of tree structure with the specific wireless node as the root and other wireless nodes than the specific wireless node as the knots and leaves in the multihop wireless network, selecting, as a target wireless node, the wireless node expected to change the number of the wireless nodes existing in a subordinate hierarchy lower than its own node from the generated network topology of tree structure, and changing, intentionally, a value of the node information sent from the target wireless node to the peripheral wireless nodes.

Because the present invention is configured in the above manner, it is possible to remove the bias of tree topology in the multihop wireless network system at a lower communication cost.

Brief description of drawings

FIG. 1 shows an example of physical configuration of a wireless sensor network in accordance with a second exemplary embodiment of the present invention;

FIG. 2 shows a connectional relation between nodes of the wireless sensor network in accordance with the second exemplary embodiment of the present invention;

FIG. 3 is a block diagram of a sensor node of the wireless sensor network in accordance with the second exemplary embodiment of the present invention;

FIG. 4 is a block diagram of a sink node of the wireless sensor network in accordance with the second exemplary embodiment of the present invention;

FIG. 5 is a time chart showing a basic operation of the wireless sensor network in accordance with the second exemplary embodiment of the present invention;

FIG. 6 shows a tree topology before correcting a bias of the wireless sensor network in accordance with the second exemplary embodiment of the present invention;

FIG. 7 is a flow chart showing a process for correcting the topological bias carried out by the sink node of the wireless sensor network in accordance with the second exemplary embodiment of the present invention;

FIG. 8 shows a configurational example of a network database possessed by the sink node of the wireless sensor network in accordance with the second exemplary embodiment of the present invention;

FIG. 9 shows the tree topology after correcting the bias of the wireless sensor network in accordance with the second exemplary embodiment of the present invention;

FIG. 10 shows a connectional relation before correcting a bias between nodes of a wireless sensor network in accordance with a third exemplary embodiment of the present invention;

FIG. 11 shows an example of topology information (before correcting the bias) possessed by a topological database of the wireless sensor network in accordance with the third exemplary embodiment of the present invention;

FIG. 12 shows an example of topology information (after correcting the bias) possessed by the topological database of the wireless sensor network in accordance with the third exemplary embodiment of the present invention;

FIG. 13 shows a connectional relation after correcting the bias between the nodes of the wireless sensor network in accordance with the third exemplary embodiment of the present invention;

FIG. 14 shows an example of physical configuration of a multihop wireless network in accordance with a first exemplary embodiment of the present invention;

FIG. 15 is a block diagram of a wireless node of the multihop wireless network in accordance with the first exemplary embodiment of the present invention;

FIG. 16 shows a topology before correcting a bias of the multihop wireless network in accordance with the first exemplary embodiment of the present invention; and

FIG. 17 shows the topology after correcting the bias of the multihop wireless network in accordance with the first exemplary embodiment of the present invention.

Exemplary embodiments

Hereinbelow, referring to the accompanying drawings, exemplary embodiments of the present invention will be explained in detail.

[A First Exemplary Embodiment]

Referring to FIG. 14, a multihop wireless network system in accordance with a first exemplary embodiment of the present invention is composed of nine wireless nodes 1 to 9. It is for the explanatory convenience that the number of wireless nodes is taken to be nine. The present invention is applicable to an arbitrary number of wireless nodes.

Each of the wireless nodes 1 to 9 has, as shown by a wireless node 10 in FIG. 15, a peripheral node information acquisition portion 11, a metric calculation portion 12, and a

superordinate destination node determination portion 13.

The peripheral node information acquisition portion 11 has a function of sending node information about its own node through broadcast to peripheral wireless nodes, and a function of acquiring peripheral node information about the node information received from the peripheral wireless nodes. In the first exemplary embodiment, the node information sent from each of the wireless nodes 1 to 9 to the peripheral nodes is composed of a node identifier for uniquely identifying its own node, and a hop number from its own node to a specific wireless node. The node information sent from a certain wireless node through broadcast is received by the other wireless nodes within the area possible for wireless communications with that wireless node as the center. Further, the peripheral node information about the node information received by each of the wireless nodes 1 to 9 from the peripheral wireless nodes is composed of the node identifier, the hop number, and a reception intensity (wireless signal intensity) when receiving the node information. Here in the first exemplary embodiment, the wireless node 1 is taken to be the aforementioned specific wireless node, while the other wireless nodes 2 to 9 are taken to be those in packet communication with the wireless node 1.

The metric calculation portion 12 has a function of calculating a metric value showing a quantity of assigning a guideline for selecting a superordinate destination node to each candidate of the superordinate destination nodes when carrying out multihop wireless communications with the specific wireless node based on the peripheral node information acquired by the peripheral node information acquisition portion 11. In the first exemplary embodiment, each of the wireless nodes 1 to 9 calculates the metric value M with the following formula by utilizing the hop number included in the received node information, and the reception intensity when

receiving that node information: M=M.sub.eh(e, h) Formula 1: where M=M.sub.eh (e, h) is a function with the hop number h and the reception intensity e as its arguments to return the metric value M with a larger value for a larger hop number h, or for a smaller reception intensity e. Further, the metric value is taken to be a smaller value for a lower communication cost of link.

The superordinate destination node determination portion 13 has a function of determining the superordinate destination node when carrying out multihop wireless communications with the specific wireless node based on the metric value calculated by the metric calculation portion 12. The superordinate destination node determination portion 13 selects a peripheral node to the superordinate destination node with the smallest metric value of the metric values calculated for each peripheral node.

On the other hand, there is another control node than the above nine wireless nodes 1 to 9. Alternatively, any one of the nine wireless nodes 1 to 9 functions as the control node. In the first exemplary embodiment, explanations are made with the example of letting the wireless node 1 function as the control node.

As shown in FIG. 15, the wireless node 1 functioning as the control node further includes a network management portion 14 in addition to the peripheral node information acquisition portion 11, the metric calculation portion 12, and the superordinate destination node determination portion 13 (other wireless nodes than the control node may not have the network management portion 14). The network management portion 14 has a function of generating a network topology of tree structure with the specific wireless node as the root and other wireless nodes as knots and leaves in the multihop wireless network, a function of selecting a predetermined wireless node as a target wireless node in the generated network topology of tree structure, and a function of intentionally changing the value of the node information sent from the target wireless node to peripheral wireless nodes.

Further, explanations will be made with respect to the function of the network management portion 14 for generating a network topology. In the first exemplary embodiment, the wireless node 1 receives packets from the other wireless nodes 2 to 9 by multihop wireless communication. At this time, a communication history is added to the receiving packets including the wireless node identifier of the source, and the wireless node identifier of the relay point. The network management portion 14 generates the above network topology based on this communication history. Alternatively, the network management portion 14 may as well generate the network topology by the following method. That is, on determining the superordinate destination node, each wireless node notifies the determined superordinate destination node to let the same recognize the existence of its own node as a child node. On receiving this notification, the superordinate destination node transfers the notification to the wireless node 1 which is the control node. Based on the information transferred in, the wireless node 1 is in a position to apprehend which wireless nodes are subordinate to which wireless nodes, that is, what the network topology is.

Further, explanations will be made with respect to the function of the network management portion 14 for selecting the target wireless node. In the first exemplary embodiment, the wireless node 1 selects a wireless node as the target wireless node with a larger number of subordinate wireless nodes compared with the other wireless nodes of the same hierarchy in the network topology of tree structure. For example, the network management portion 14 investigates the number of subordinate nodes (the number of child and grandchild nodes including child nodes and grandchild nodes) of each wireless node belonging to each hierarchy other than the root hierarchy of the network topology, and compares the average or medium value, standard deviation, maximum value, minimum value, and the like, so as to detect the biased places of the number of subordinate nodes. For example, if there are four sensor nodes A, B, C and D in a certain hierarchy, and the number of subordinate nodes is, respectively, 10 units for A, 33 units for B, 9 units for C and 5 units for D, then the average number can be found to be approximately 15 units. At this time, if it is predetermined to detect places with a node number of twice the average value or more as the biased places, for example, then node B is detected as the target wireless node.

Further, explanations will be made with respect to the function of the network management portion 14 for changing the node information. The network management portion 14 carries out a control process to change the node information sent from the target wireless node to the peripheral wireless nodes such that the metric value calculated with the target wireless node as a candidate for the superordinate destination node may become larger in terms of the communication cost of link. In particular, it sends a message of change request to the target wireless node, and receives the replay. In the first exemplary embodiment, the node information sent from a wireless node to the peripheral wireless nodes is the node identifier and hop number, and the metric value is calculated by the above Formula 1. Therefore, the network management portion 14 changes the hop number sent from the target wireless node to the peripheral wireless nodes, to a larger number than the actual hop number. The extent of increasing the number is arbitrary. It may be increased by a predetermined number (one hop number for example), or be rounded off after adding a predetermined fraction (50% for example) to the actual hop number, etc.

Next, operations of the first exemplary embodiment will be explained.

By the peripheral node information acquisition portion 11, the wireless nodes 1 to 9 send, periodically for example, the node information including the hop number from the wireless node 1 to the peripheral wireless nodes. In the initial state, because only the wireless node 1 recognizes the hop number from the wireless node 1, the wireless node 1 sends the hop number 0 to the peripheral wireless nodes through broadcast. Now, suppose that the wireless nodes having received this broadcast are two wireless nodes: the wireless node 2 and the wireless node 3. The wireless nodes 2 and 3 utilize the metric calculation portion 12 to calculate the metric value with the wireless node 1 as a candidate for the superordinate destination node based on the hop number 0 received from the wireless node 1 and the reception intensity, and determine the wireless node 1 to be the superordinate destination node by the superordinate destination node determination portion 13. At this time, suppose that the metric value of the wireless node 1 calculated by the wireless nodes 2 and 3 is 2.0 and 3.0, respectively.

Next, the wireless nodes 2 and 3 send the hop number 1 up to the wireless node 1 determined to be the superordinate destination node to the peripheral wireless nodes through broadcast. Suppose that the wireless nodes having received the broadcast from the wireless node 2 are five units except the wireless node 1: the wireless nodes 4, 5, 6, 7, and 8, while the wireless nodes having received the broadcast from the wireless node 3 are three units except the wireless node 1: the wireless nodes 7, 8, and 9.

The wireless nodes 4, 5 and 6 calculate the metric value with the wireless node 2 as a candidate for the superordinate destination node based on the hop number 1 received from the wireless node 2 and the reception intensity, and determine the wireless node 2 to be the superordinate destination node. At this time, suppose that the metric value of the wireless node 2 calculated by the wireless nodes 4, 5 and 6 is 5.0, respectively. In the same manner, the wireless node 9 calculates the metric value with the wireless node 3 as a candidate for the superordinate destination node based on the hop number 1 received from the wireless node 3 and the reception intensity, and determines the wireless node 3 to be the superordinate destination node. At this time, suppose that the metric value of the wireless node 3 calculated by the wireless node 9 is 6.0.

On the other hand, the wireless nodes 7 and 8 receive the node information from both the wireless nodes 2 and 3. The wireless nodes 7 and 8 calculate the metric values with the wireless nodes 2 and 3 as candidates for the superordinate destination nodes, respectively, and determine the one with the smaller metric value to be the superordinate destination node. Now, suppose that the metric values of the wireless node 2 and the wireless node 3 calculated by the wireless node 7 are 4.5 and 5.5, respectively, while the metric values of the wireless node 2 and the wireless node 3 calculated by the wireless node 8 are 5.0 and 5.5, respectively. That is, although the hop numbers are both 2, the different reception intensities bring about such results. Hence, both the wireless nodes 7 and 8 determine the wireless node 2 to be the superordinate destination node.

Thereafter, from the wireless nodes 4 to 9, the node information is sent to the peripheral nodes and, from the wireless nodes 1 to 3, the node information is sent again to the peripheral nodes. The wireless nodes having received the above node information repeat the calculation of the metric values, and the determination of the superordinate destination node. Here, suppose that there is no change in the result of determining the superordinate destination node. At this stage, the network topology of tree structure is shown in FIG. 16, with the wireless node 1 as the root, and the wireless nodes 2 to 9 as the knots or leaves.

In FIG. 16, the solid lines linking upper and lower wireless nodes show that lower wireless nodes select upper wireless nodes as superordinate destination nodes. Further, the dashed lines linking upper and lower wireless nodes show that although lower wireless nodes select an upper wireless node as a candidate for the superordinate destination node, because there is another candidate for the superordinate destination node with a smaller metric value, this candidate is discarded. Further, the numerical values assigned to the solid lines and the dashed lines show the metric values calculated by the lower wireless nodes taking the upper wireless nodes as the objects. Referring to FIG. 16, the number of the subordinate nodes of the wireless node 2 is five, while the number of the subordinate nodes of the wireless node 3 in the same hierarchy is one. This indicates that there is a large bias.

The network management portion 14 of the wireless node 1 which is the control node generates the network topology as shown in FIG. 16, and selects the wireless node 2 as the target wireless node with a larger number of subordinate wireless nodes compared with the other wireless nodes in the same hierarchy. Then, the network management portion 14 of the wireless node I sends a message of change request to the wireless node 2 for changing the hop number to 2, for example, which is sent from the wireless node 2 to the peripheral wireless nodes, as the result of adding a predetermined number 1 to the actual hop number 1. On receiving the message of change request, the peripheral node information acquisition portion 11 of the wireless node 2 sends the hop number 2 different from the actual number to the peripheral nodes after sending a reply to the wireless node 1. On the other hand, the wireless node 3 still sends the actual hop number 1 to the peripheral nodes.

Suppose that the wireless nodes having received the broadcast from the wireless node 2 are, in the same situation as before, are five units except the wireless node 1: the wireless nodes 4, 5, 6, 7 and 8, while the wireless nodes having received the broadcast from the wireless node 3 are three units except the wireless node 1: the wireless nodes 7, 8 and 9.

The wireless nodes 4, 5 and 6 calculate the metric value with the wireless node 2 as a candidate for the superordinate destination node based on the hop number 2 received from the wireless node 2 and the reception intensity, and determine the wireless node 2 to be the superordinate destination node. At this time, suppose that the metric value of the wireless node 2 calculated by the wireless nodes 4, 5 and 6 is 6.5, respectively. In the same manner, the wireless node 9 calculates the metric value with the wireless node 3 as a candidate for the superordinate destination node based on the hop number 1 received from the wireless node 3 and the reception intensity, and determines the wireless node 3 to be the superordinate destination node. At this time, suppose that the metric value of the wireless node 3 calculated by the wireless node 9 is 6.0.

On the other hand, the wireless nodes 7 and 8 receive the node information from both the wireless nodes 2 and 3. The wireless nodes 7 and 8 calculate the metric values with the wireless nodes 2 and 3 as candidates for the superordinate destination nodes, respectively, and determine the one with the smaller metric value to be the superordinate destination node. Now, suppose that the metric values of the wireless node 2 and the wireless node 3 calculated by the wireless node 7 are 6.0 and 5.5, respectively, while the metric values of the wireless node 2 and the wireless node 3 calculated by the wireless node 8 are 6.5 and 5.5, respectively. That is, even though the reception intensities are the same as in the previous situation, the hop numbers are 2 hops for the wireless node 2 but 1 hop for the wireless node 3. This difference brings about such results. Hence, both the wireless nodes 7 and 8 determine the wireless node 3 to be the superordinate destination node.

Thereafter, from the wireless nodes 4 to 9, the node information is sent to the peripheral nodes and, from the wireless nodes 1 to 3, the node information is also sent to the peripheral nodes. The wireless nodes having received the above node information repeat the calculation of the metric values, and the determination of the superordinate destination node. Here, suppose that there is no change in the result of determining the superordinate destination node. At this stage, the network topology of tree structure changes as shown from FIG. 16 to FIG. 17, with the wireless node 1 as the root, and the wireless nodes 2 to 9 as the knots or leaves.

Referring to FIG. 17, the number of subordinate nodes of the wireless node 2 changes from 5 to 3 and, as a result, is equal to the number of subordinate nodes of the wireless node 3 in the same hierarchy.

In this manner according to the first exemplary embodiment, it is possible to remove the bias of tree topology in the multihop wireless network system at a lower communication cost. In the above example for instance, the message of change request is sent once from the wireless node 1 which is the control node to the wireless node 2, and the reply is sent once from the wireless node 2 to the wireless node 1, Since from the wireless node 1 to the wireless node 2 is one hop, the total transmissions are two. In contrast to this, if the wireless node 1 sends a message to the wireless nodes 7 and 8 for the request to shift the superordinate destination node from the wireless node 2 to the wireless node 3, and the wireless nodes 7 and 8 send a reply to the wireless node 1, then since from the wireless node 1 to the wireless nodes 7 and 8 are two hops, the total transmissions are eight.

In the first exemplary embodiment, although the node information sent from a wireless node to the peripheral nodes is composed of the node identifier and the hop number, instead of the hop number or in addition to the hop number, other data may as well be utilized. For example, instead of the hop number or in addition to the hop number, the number of child nodes may as well be utilized, which is the number of the wireless nodes selecting its own node as the superordinate destination node. In this case, the metric value M may be calculated by the following formulas: M=M.sub.ek (e, k), Formula 2: and M=M.sub.ehk(e, h, k) Formula 3: where M=M.sub.ek (e, k) is a function with the reception intensity e and the child node number k as its arguments to return the metric value M with a larger value for a larger child node number k, or for a smaller reception intensity e, while M=M.sub.ehk(e, h, k) is a function with the reception intensity e, the hop number h, and the child node number k as its arguments to return the metric value M with a larger value for a larger hop number h and child node number k, or for a smaller reception intensity e. Then, in these cases, objects to be intentionally changed to different values from the actual values may be taken as the hop number and child node number, or just the child node number.

Further, in the first exemplary embodiment, although the node information sent from a wireless node to the peripheral nodes is composed of the node identifier and the hop number, instead of the hop number or in addition to the hop number, the metric value of a superordinate destination node may as well be utilized, which is calculated when its own node determines the superordinate destination node. In FIG. 16 for example, the wireless node 2 sends the node information including the metric value 2.0 of the wireless node 1 to the peripheral nodes. In this case, the metric value M may be calculated by the following formulas: M=M.sup.em (e, m), Formula 4: and M=M.sup.ehm (e, h, m) Formula 5: where M=M.sup.em (e, m) is a function with the reception intensity e and the metric value m as its arguments to return the metric value M with a larger value for a larger metric value m, or for a smaller reception intensity e, while M=M.sup.ehm(e, h, m) is a function with the reception intensity e, the hop number h, and the metric value m as its arguments to return the metric value M with a larger value for a larger hop number h, for a larger metric value m, or for a smaller reception intensity e. Then, in these cases, objects to be intentionally changed to different values from the actual values may be taken as the hop number h and metric value m, or just the metric value m.

Further, in calculating the metric value, the abovementioned Formulas 1 to 5 may as well be transformed as into the following formulas by utilizing a handicap value which is a weighting parameter value: M=M.sub.eh (e, h)+.alpha., Formula 1': M=M.sub.ek (e, k)+.alpha., Formula 2': M=M.sub.ehk (e, h, k)+.alpha., Formula 3': M=M.sub.em (e, m)+.alpha., Formula 4': and M=M.sub.ehm (e, h, m)+.alpha. Formula 5':

In the above formulas 1' to 5', .alpha. represents a handicap value. The initial value of a handicap value .alpha. is zero. Then, in these cases, in order to remove the bias of tree topology, the handicap value .alpha. may be set or changed. That is, in order to remove the bias of tree topology, the wireless node 1, which is the control node, sets or changes the handicap value of the target wireless node 2, and sends the changed handicap value to the wireless node 2 with the message of change request. On receiving the message of change request, the peripheral node information acquisition portion 11 of the wireless node 2 sends the handicap value as one piece of the node information to the peripheral nodes after sending a reply to the wireless node 1. The peripheral wireless nodes having received this node information utilize the latest changed handicap value included in the received node information to calculate the metric value with the wireless node 2 as a candidate for the superordinate destination node.

Further, although there is a unique specific wireless node in the first exemplary embodiment, there may as well be two or more specific wireless nodes. In this case, other wireless nodes than the specific wireless nodes do not communicate with the plurality of specific wireless nodes but communicate with one specific wireless node with a smaller metric value. Therefore, when there are a plurality of specific wireless nodes, the same number of tree topologies are formed as that of the specific wireless nodes. Then, if bias occurs in the number of wireless nodes attributive between the plurality of tree topologies, then a control for removing the above bias may as well be carried out by intentionally changing the data such as the hop number, child node number, metric value, handicap value and the like included in the node information sent from a specific wireless node to the peripheral nodes.

Further, in the first exemplary embodiment, by comparison with other wireless nodes in the same hierarchy of topology, the wireless node 2 with a larger number of subordinate wireless nodes is selected as the target wireless node. However, by comparison with other wireless nodes in the same hierarchy of topology, the wireless node 3 with a smaller number of subordinate wireless nodes may as well be selected as the target wireless node. In this case, the hop number sent by the wireless node 3 to the peripheral nodes may be changed such that the metric value may become smaller in terms of the communication cost of link, calculated with the wireless node 3 as a candidate for the superordinate destination node (that is, a smaller hop number than the actual number is sent to the peripheral nodes). When the hop number is set to be a smaller value than the actual hop number, because it is possible to give rise to a loop in constructing the topology, it is necessary to incorporate a process for avoiding the loop. In the present invention, however, because the process for avoiding the loop is beyond the scope, its details will not be included in the description.

[A Second Exemplary Embodimen]

Next, a second exemplary embodiment of the present invention will be explained in detail.

[Background]

For wireless sensor networks applied to environment measuring systems collecting environment information such as temperature, humidity, and the like, it is necessary to collect the data sensed by each node (sensor node) into one node (sink node). Therefore, as a network topology, it is common to construct a tree topology with the sink node as the apex. This tree topology is generally constructed by a routing protocol suitable for wireless multihop networks.

In the routing protocols utilized in wireless multihop networks, there are proactive routing protocols and reactive routing protocols. The representative example of proactive protocols is the OLSR (Optimized Link State Routing) protocol. The representative example of reactive protocols is the AODV (Ad hoc On-Demand Distance Vector) protocol, and the DSR (Dynamic Source Routing) protocol. The OLSR, AODV and DSR are protocols defined by IETF (Internet Engineering Task Force).

In these routing protocols, based on the reception intensity and hop number, the metric value is calculated to indicate the cost up to the sink node, and a network topology is constructed based on the calculated metric value. Therefore, when applying these routing protocols in wireless sensor networks, each sensor node selects the path (route) up to the sink node with the minimum communication cost of link to construct the network topology.

Further, since there is no prescription for the optimization and equalization method of network topology, individual measures are taken for different systems, or topologies generated by routing protocols are utilized as they are. Therefore, there are problems as follows.

The first problem is that because a sensor node selects the minimum path up to the sink node, bias occurs in the number of nodes connected in the network topology, whereby the communication band cannot be utilized effectively.

The second problem is that in order to equalize the network topology, when sending a message of destination change to each node, it is necessary to augment the process for adjusting the participant nodes as for how to move (concerning the topological shape) what number of units to what places, thereby resulting in the increase of load on the management server.

The third problem is that in order to equalize bias in the network topology, it is necessary to send a message of destination change to each node, thereby compressing the communication band.

An object of the second exemplary embodiment is to provide a method for realizing the equalization of tree topology at a low cost (a small amount of communications and calculations) in a wireless sensor network.

[Outline]

In the second exemplary embodiment, in constructing a tree topology for a wireless sensor network system for collecting sensor data, when bias occurs in the number of nodes constituting every branch of the tree topology, the node number is equalized with a small amount of communications and calculations to disperse the load.

Every node constituting the wireless sensor network constructs a tree topology clustered autonomously with the sink node as the apex by a peripheral node information acquisition unit for sending node information and acquiring peripheral node information, a metric calculation unit for calculating a metric value indicating the cost for reaching the sink node for collecting sensor data, and a destination node determination unit for determining the connection node based on the metric value calculated by the metric calculation unit. At the time, bias may occur in the number of subordinate nodes among the respective branches within the tree topology. In the second exemplary embodiment, for the purpose of removing the bias in the number of subordinate nodes among the respective branches, the number of subordinate nodes is equalized in promoting the motion of the nodes among the respective branches by compulsorily changing the hop number up to the sink node sent from a predetermined sensor node to the peripheral nodes.

In this manner, in the network autonomously constructing a network topology according to the second exemplary embodiment, by intentionally changing the hop number utilized to construct the topology, it is possible to disperse the load in equalizing the number of nodes of tree topology in the wireless sensor network with a small amount of calculations and communications. Hereinbelow, the configuration and operation of the second exemplary embodiment will be explained.

[Configuration]

FIG. 1 shows a physical configuration utilizing a sink node and sensor nodes in accordance with the second exemplary embodiment. Referring to FIG. 1, the sink node 201 is a device for collecting the information sensed by sensor nodes 202 to 211 from these sensor nodes 202 to 211. The sensor nodes 202 to 211 are installed indoors or outdoors to sense external information, and send the sensed information to the sink node 201. Further, in FIG. 1, the zigzag lines linking respective nodes show a relationship of carrying out wireless communications.

The description continues in the full USPTO document.